1. Field of the Invention
The present invention relates to electrical filter networks for filtering selected frequencies. More specifically, the present invention relates to notch filter networks which utilize, in combination, a high Q cavity filter and a low Q lumped constant filter network to produce an electrical filter network of improved characteristics. The present invention also relates to multicouplers such as diplexers and duplexers which include the filter network of the present invention. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. The Prior Art
In my prior patents, Ser. Nos. 3,717,827 and 3,815,137 issued on Feb. 20, 1973 and June 4, 1974, respectively, interference problems in the field of radio communications were discussed. Briefly, these problems involve the simultaneous utilization of one antenna or transmission line with two or more transmitting and receiving pieces of equipment operating at carrier signals of different frequencies such as are found in diplexers and duplexers. In a diplexer at least two receivers or two transmitters share an antenna. In a duplexer, which is the more difficult of the two, at least one receiver and one transmitter share the same antenna. In order to properly isolate the various pieces of equipment from one another, a number of filter sections are commonly utilized. These filter sections each reject a first frequency and pass a second frequency. It is desirable for these filter sections to be easily tuneable to vary either the pass or reject frequencies. It is also desirable, in certain applications, to have as broad a reject band as possible to reduce the number of filters required to properly isolate the equipment. The goal of attaining a broad reject band, however, should not sacrifice the selectivity of the filter so as to adversely effect the proximity of the reject band and the pass band which should be as close together as possible. Furthermore, it is always commercially desirable for the filter device to be of simple, straight forward construction so that it might be easily manufactured at relatively small cost. It is also desirable that the filter have a high operating efficiency.
Other filtering devices are known which satisfy these objects to one degree or another. One such filtering device is described in U.S. Pat. No. 3,876,963 issued on Apr. 8, 1975 to Gerald Graham. Still other notch filtering devices may be found described in U.S. Pat. Nos. 3,680,011; 3,697,903; 3,967,102; and 3,925,739. Each of these devices has one or more drawbacks. Therefore, it is apparent that an inexpensive and flexible notch filter is needed to adequately solve many of the problems of radio frequency interference found in multicouplers.